docs: def __add_symbols(self, cmd):
          """
          Add all additional defined and undefined symbols.
  
          """
  
          if self.__config.define_symbols:
              symbols = self.__config.define_symbols
              cmd.append(''.join(
                  [' -D"%s"' % def_symbol for def_symbol in symbols]))
  
          if self.__config.undefine_symbols:
              un_symbols = self.__config.undefine_symbols
              cmd.append(''.join(
                  [' -U"%s"' % undef_symbol for undef_symbol in un_symbols]))
  
          return cmd